The present invention relates to modified stainless steel powders and compacts formed therefrom, and more particularly to improving the processability of such powders and compacts and improving corrosion resistance properties.
It is known in the art that the corrosion resistance of stainless steel powders can be improved by making tin additions to the stainless steel powders. U.S. Pat. No. 4,240,831 to Ro et al. teaches a process for improving the corrosion resistance of stainless steel powders through the addition of an effective proportion of a modifier metal selected from the group consisting of tin, aluminum, lead, zinc, magnesium, rare earth metals and like metals.
U.S. Pat. No. 4,314,849 to Ro et al. also teaches that the corrosion resistance of stainless steel powder compacts can be improved if they contain tin and silicon. Ro et al. aver that the corrosion resistance can be maximized if compacts formed from such modified stainless steel powders are sintered at temperatures in excess of 2300.degree. F. in highly reductive atmospheres until the ratio of Sn:Si on the surface of the compact is at least about 1:1. U.S. Pat. No. 4,420,336 to Klar et al. relates to a foraminous body having improved corrosion resistance to aqueous nitric acid. The foraminous body is formed of tin-containing water atomized, compacted and sintered austenitic stainless steel alloy powder compacted and sintered to less than 80% of theoretical density. It is also formed of a prealloyed stainless steel alloy powder containing from 0.1% to 10% by weight tin and, optionally, from 0.5% to 5% copper.
U.S. Pat. No. 4,662,939 to Reinshagen, assigned to the assignee of the present invention, teaches that the corrosion resistance of stainless steel powder moldings can be improved by combining the powder before molding with about 8% to 16% by weight of an additive consisting essentially of about 2 to 30% by weight of tin and 98 to 70% by weight of copper and/or nickel. It has been found in practice that parts manufactured from this composition, while demonstrating excellent corrosion resistance properties, grow on sintering. As a result, these parts have limited acceptance since they typically do not meet required dimensional tolerances. Parts manufactured from stainless steel powders which exhibit improved corrosion resistance and which are capable of meeting required dimensional tolerances are in demand.